This invention relates to the preparation of rigid cellular polyurethanes. This invention further relates to novel gel catalyst compositions for preparing rigid cellular polyurethanes. Using these catalyst compositions the rate of the isocyanate-polyol reaction can be controlled within a wide range not heretofore possible using combinations of prior art catalysts.
Rigid polyurethane foams are used as insulating materials for numerous end-use applications, including insulation for structures and containers. For some of these applications, such as spraying, it may be desirable that foam formation and solidification take place as rapidly as possible once the polyfunctional isocyanate and the polyol are combined. For other applications, wherein the formulation must fill large cavities or complex molds, it would be desirable to delay initiation of the isocyanate-polyol reaction until the mixture of reagents has completely filled the cavity or mold. Once this has occurred, foaming and solidification should occur as rapidly as possible to minimize residence time in a mold. Using conventional gel catalysts for rigid polyurethane foams, which include tertiary amines used either alone or in combination with organotin compounds, it is not feasible to control the rate of the isocyanate-polyol reaction over the range required for the various applications of rigid polyurethane foams. The reaction when catalyzed by amine is relatively slow, and the rise time (the time required for the foam to reach its final height) may be as long as 3 or 4 minutes. The rise time is decreased by several orders of magnitude if the amine is used in combination with an organotin compound such as dibutyltin dilaurate. This combination of an amine with an organotin compound is synergistic with the result that the reaction mixture may solidify too rapidly for certain applications, even when the concentration of the organotin compound is reduced to the lowest practical level.
It is an objective of this invention to define a catalyst composition for rigid cellular polyurethanes that will enable the rate of the isocyanate-polyol reaction and the time required to initiate this reaction to be varied within wide ranges. This objective can be achieved using a gel catalyst composition containing an antimony carboxylate, a potassium carboxylate and a zinc carboxylate. This composition can be used along or in combination with the amines and organotin compounds conventionally employed as catalysts for rigid cellular polyurethanes.